Belgium on Monday slammed as "unacceptable" a plan by Hungary's anti-immigration government to send asylum seekers to Brussels, where the European Union is headquartered, on buses.

Prime Minister Viktor Orban -- who is staunchly anti-immigration -- has been at loggerheads with the EU over migration and refugee policy.

In July, Budapest said it would not pay a 200 million-euro-fine ($220 million) as well as a daily one-million-euro penalty imposed by the European Court of Justice for violating asylum rules, claiming the bloc was exerting "political pressure" on Hungary.

Last week, Hungary showed off yellow buses it said would transport asylum seekers arriving at its southern border with Serbia straight to Brussels and free of cost.

Deputy interior minister Bence Retvari on Friday claimed the EU wanted "to force Hungary to let in illegal migrants who have been intercepted", adding that Budapest would "offer these illegal migrants a free, voluntary trip to Brussels".

"If Brussels wants illegal migrants, Brussels can have them," said Retvari, according to state television.

Belgium's secretary of state for asylum and migration Nicole de Moor on Monday called Hungary's "threat to send refugees to Brussels... unacceptable".

"Unilateral shifting of responsibility through threats of this kind undermines solidarity and cooperation within the Union," she said in a statement.

Questioned on Budapest's proposal last month, the European Commission declined comment, saying it was used to "strong statements on this policy field coming from Hungary".

Orban continues to vaunt Hungary's tough line on migration, erecting a razor wire fence along its southern frontier during the height of the 2015 migrant crisis and making it virtually impossible for refugees to seek asylum there.

If Budapest refuses to pay the fine imposed by the bloc's top court, the European Commission will deduct the money from the funds earmarked for Hungary it had not yet paid out.